1. Field of the Invention
The present invention relates to a recording head system for an ink jet recording apparatus and method for driving the same, which recording apparatus includes a so-called full-line recording head comprising ink nozzles arranged along the line across a recording medium.
2. Description of the Prior Art
An ink jet recording apparatus is a recording system which performs recording by forming ink droplets by some means, and depositing the ink droplets on a recording medium such as recording paper. Among these ink jet recording apparatuses, one which employs thermal energy to form droplets by discharging liquid has superior characteristics so that high resolution, high quality images are quickly obtained. This is because a high density multi-nozzle system can be easily implemented in this apparatus.
This type of ink jet recording apparatus comprises a recording head for a line printing, that is, a so-called full-line recording head including ink discharging portions such as ink nozzles, ink discharging ports or ink discharging orifices (henceforth they are referred to as "ink nozzles") aligned along the full line across a recording medium. This recording head comprises a plurality of liquid droplet forming means, each of which includes an electrothermal energy converting element, and a plurality of integrated circuits (driving ICs) for driving the electrothermal energy converting elements, wherein the liquid droplet forming means and the integrated circuits are formed on a single substrate. Here, the liquid droplet forming means are for discharging ink droplets from the nozzles by providing the ink with thermal energy, and the electrothermal energy converting elements heat the ink by electric current pulses supplied to the elements.
FIG. 1 shows a full-line recording head including ink nozzles arranged along the line across a recording medium.
Electrothermal energy converting elements 1, which are equally spaced, are formed together with wires, on a substrate 1 of silicon or the like through the processes similar to those employed for fabricating semiconductor devices. Separation walls 14 are formed between respective two adjacent electrothermal converting elements 1 by depositing resin layers. A flat liquid-passage-forming material 16 is joined on the separation walls 14, followed by attaching a top plate 17 of glass or the like. Thus, nozzles 12, liquid passages 13 and a common liquid chamber 15 are formed.
FIG. 2 illustrates a drive control circuit for controlling the recording head shown in FIG. 1.
A drive IC 5 is provided for each block comprising n (n=64, for example) electrothermal energy converting elements 1. Recording data SI consisting of the number of bits identical to the number of the electrothermal energy converting elements 1 are sequentially transferred to serially connected shift registers 4 in respective drive ICs 5 in synchronism with a data transfer clock (SCLK). After all the data are inputted to the shift registers 4, the data are read into latches 3 by the latch signal LAT. Subsequently, the drive ICs are sequentially enabled by D flip-flops 22 in response to the input of a time-division driving signal EI and a time-division driving signal transfer clock ECLK. Thus, only the electrothermal energy converting elements 1 associated with the ON-state record data of the enabled drive IC 5 are selectively supplied with currents during the ON interval of a current supply interval setting signal BEI, thereby discharging ink. This operation is called a block drive whose timings are illustrated in FIG. 3.
In this apparatus, recording is performed by directly discharging ink from the ink nozzles of the recording head by utilizing the pressure of the bubbles generated in the ink by supplying the electrothermal energy converting elements 1 with electric current. Accordingly, it is necessary to maintain the ink in such a state that it can be discharged without fail. The ink discharge, however, sometimes becomes unstable. The reason for this is as follows: The ink discharge, which is performed by supplying current to the electrothermal energy converting elements 1, induces changes in pressure, and the pressure changes may sometimes cause oscillation of the ink in the adjacent passages 13 through the common liquid chamber 15. As a result, when the electrothermal energy converting elements 1 placed in the adjacent passages 13 are continuously driven, the discharge becomes unstable because of the pressure changes. This leads to the changes in the volume of discharged ink, resulting in the unevenness of density of a recorded image. Such variations in the discharged volume due to the pressure changes of the ink increase consistently with the number of bits driven simultaneously, and decrease with the distance from the nozzle to the location where the pressure changes take place. Thus, the pressure changes are largely affected by the geometry of the common liquid chamber communicating to each nozzle.
In addition, since the interval between the drive timings of two adjacent blocks is constant regardless of the recording data, the bubble pressure varies when the recording frequency is high, that is, when the number of elements continuously and simultaneously driven is large. This variation in pressure transfers to the adjacent liquid passages 13 via the common liquid chamber, and causes oscillation of the ink. As a result, the discharge becomes unstable, and the volume of discharged ink varies. This presents a problem that the unevenness of density takes place in a recorded image.
Furthermore, there is another problem that the unevenness of density occurs because the variation in the discharged ink volume is greater at the center of the recording head than at the ends thereof.
For these reasons, it was necessary to enlarge the common liquid chamber so that the changes in the ink pressure have little influence on the ink discharging operation from the nozzles, or to lengthen the interval to drive the adjacent electrothermal energy converting elements. This hinders the high speed recording and the reduction in size of the recording head.
To overcome such a problem, it might be possible to drive all the electrothermal energy converting elements simultaneously. The current flowing through a single electrothermal energy converting element 1, however, is rather large ranging from several tens to hundreds of milliamperes. Consequently, the total sum of the current required for driving all the elements grows very large, which is unacceptable from the viewpoint on shrinking the power supply and recording head. This is the reason why the block drive has been employed in which the electrothermal energy converting elements are divided into a plurality of blocks, and the blocks are driven in time sharing fashion.